younger sister, sister-in-law
娣 centers on a female relative in a junior position: a younger sister or a sister-in-law, specifically a younger brother's wife.
娣 combines 女 (woman) with 弟 (younger brother), likely indicating a female relative associated with a younger brother, such as a younger sister or sister-in-law.
A woman 女 beside a younger brother 弟: think of a younger sister or the wife of a younger brother, both junior female relatives.
For テイ, imagine a younger sister named Tay who is always by her brother's side: Tay -> テイ, and she is the 娣.